What did Mao's Long March accomplish? Why was it successful?
The survival of the Communist Party.
Although suffering appalling losses the Communist Party emerged from the Long March re-organised and began to fight a long and eventually successful war against the nationalists. This culminated in Mao declaring the new Communist Government of China in October 1949.
It also served as a very important example of symbolism for the communist party after it gained power and particularly in cultivating the myth of Mao.